Low starting torque for motor and capacitor size

[Geoffm]

I recently bought a new to me, 2021 vintage saw bench with a 3hp 1ph motor. 8t had a known fault that it would not always spin up when you turned it on. If given a flick it would go. If had been running it would start ok. Leave it for a few hours or the next day, it would not start. It is like it has really low start torque.
I thought it might be the 35mf capacitor so replaced that but no difference.
Would a larger capacitor hepp? Any other ideas? There doesn't seem to be any mechanical issues and everything moves and sounds ok.
I could replace the motor but I'm sure it a simple problem

 

[Bornfree]

Does it have two capacitors one large and one small?

 

[Anguz]

is there a centrifugal switch to cut out the cap when its running?, it may be stuck open

 

[AVI_8]

Has the motor definitely only got 1 capacitor? Often there’s 2, a start capacitor which is larger and a smaller run capacitor.

 

[AVI_8]

You guys beat me to it there, good point about the centrifugal switch, if you unload the motor by removing the belt and switch the motor on you should hear the switch disengage when the motor slows down after it’s turned off if it has one.

 

[Geoffm]

Only one capacitor which is 35mf. Google tells me that 35mf is aboutright for a run cap. I can't hear any clicking of a start centrifugal switch.

 

[Agroshield]

What is the supply like?

If you are on the end of a 50m long 1.5mm extension lead, it will not have as much grunt as a 2" dia. per core cable plugged directly into Hinkley Point.

 

[Geoffm]

What is the supply like?

If you are on the end of a 50m long 1.5mm extension lead, it will not have as much grunt as a 2" dia. per core cable plugged directly into Hinkley Point.

Supply should be ok. The workshop was built 10 years ago with brand new wiring and 3ph. Saw has almost extension cord to the wall socket. A different cord and socket make no difference.

 

[carbon]

If the motor is 3hp and 1400 rpm I’d expect a 75uf or 80uf cap to be fitted. If it’s 2800rpm then approx 40uf

If the motor has a data plate could you post a picture of it

 

[Stupoty]

You guys beat me to it there, good point about the centrifugal switch, if you unload the motor by removing the belt and switch the motor on you should hear the switch disengage when the motor slows down after it’s turned off if it has one.

If it has a centrifugal switch that is quiet you can often take the belts off and spin it by hand using the pulley on the motor shaft when the powers off and listen for a slight tick or click.

(or open the rear bell and have a look if that is easy to do).

Stu

 

[Geoffm]

I opened up the side of the saw today to measure the motor size and have a look. I took the end off, and there is no sign the fan is rubbing.
Still got me bluffed.

 

[Geoffm]

The original motor starts fine on the bench, but has zero starting torque, so even though the motor turns easily, as does the saw spindle, it stalls.

Google tells me it’s a "polyphase" or "2 phase" motor. There is no start circuit and start capacitor, rather it is like a 3ph motor with a capacitor for the 2nd phase and hence low starting torque. It shouldn’t need much to start a saw blade, but whatever it has, is not enough.
I gave up and spend $NZ230 (115 pounds) on a new motor. This is China's Finest cap start, cap run, and seems to run ok. Sod of a thing to fit into the saw cabinet and get it bolted up and lined up. I was worried it might be a bit long to fit with the panels on the side. I had to turn a recess in the original pulley to get it close enough to the motor housingto lin up with the saw spindle.
Took a while to sort out the switch as it has 5 pins and had 4 core cable to the motor. The 5th pin is the power failure widget, and the 4th wire takes power back form the motor live side to the switch, rather than a simple loop at the switch.
Black motor is the original one, blue motor is the replacement.

 

[hunter27]

I had the same problem on my table saw and asked the same question here a few years ago. Mine is direct drive but if the blade didn't start which was quite regularly the 13A fuse blew within a couple of seconds. Pushing the blade around with a stick always started it spinning if you were quick before the fuse blew. I replaced the external capacitor as advised which mostly solved this problem but not completely. I still use the saw regularly and it works well but has almost no torque on start up.

 

[6ply]

I opened up the side of the saw today to measure the motor size and have a look. I took the end off, and there is no sign the fan is rubbing.
Still got me bluffed.
View attachment 400218View attachment 400219View attachment 400220

Nothing to do with the starting torque, but you need a cowl / fan cover on that motor. The cowl is an integral part of the air flow cooling system. I was going to offer you one but then noticed you are in NZ.

 

[Geoffm]

Nothing to do with the starting torque, but you need a cowl / fan cover on that motor. The cowl is an integral part of the air flow cooling system. I was going to offer you one but then noticed you are in NZ.

It has the cowl. You can see it in the last photo on the bench with the replacement. The photo on the saw had the cowl removed as I was trying to see if it was rubbing, or there was an accessible start switch